Typically, integrated waveguide circuits have flat cut or polished ends. The coupling efficiency between these waveguide circuits and a laser diode is low, typically less than 10%, because of the divergence of light from a laser diode. Discrete ball lenses or cylindrical bar lenses have been used to improve the coupling between laser diodes and waveguides but the discrete lenses are difficult to handle because of their small sizes. With the current invention, the lens is directly formed from the waveguide which eliminates the trouble of handling and aligning the lens with the waveguide.
Optical fibers with lens ends have been made to improve the coupling between laser diodes and fibers, but no planar process channel waveguides have been made with lens ends. Planar processed channel waveguides have different applications and functions than optical fibers.
In U.S. Pat. No. 5,064,266, having the common assignee of the present invention, lenses are formed monolithically with channel waveguides on the same substrate. The lenses are formed from "positive type" waveguides and the waveguide core is exposed to air during heating. Therefore, the core of the waveguide is always rounded while forming the lenses. With the present invention, the waveguide core other than the ends is enclosed in cladding during heating. The waveguide core retains its cross-sectional shape while the ends are made into lenses by heating. This enables the waveguide core to be of various shapes thus allowing many types of circuit design details to be accomplished.
The present invention offers other advantages over previously known methods and devices. The present invention enables better and relatively easier optical coupling from light sources, such as laser diodes, to waveguides and from waveguides to light detectors, such as photo diodes. This invention will be particularly useful in fiber optic communication and optoelectronic integration. The integrated waveguide circuits made by the present invention have substantially cone-shaped lens ends with a smooth surface which improve the waveguide's optical coupling to light sources and detectors.
The method of the present invention to make integrated optical circuits having waveguide ends of lens geometry is based on the principal that the etching rate of doped material varies with the type of dopants and the dopant concentration. In accordance with the present invention, waveguide circuits are formed on a substrate and the waveguide ends are shaped into lens form. The formation of waveguide circuits on a substrate includes the formation of one or more channels in lower cladding, wherein the channels define a waveguide circuit configuration. The substrate may itself be lower cladding or lower cladding may be deposited on the substrate. In either situation, core material is deposited over the lower cladding to fill the channel with the core material, wherein the core material has a relatively higher refractive index than the lower cladding. Excess core material is removed to leave the channels filled with the core material and upper cladding is deposited over the channels that are filled with the core material.
The waveguide ends may be shaped into lens form or geometry by etching the ends of the waveguide to form a protrusion from the ends of the waveguide with a gradually reduced cross-sectional area. The waveguide is heated to or above a softening temperature resulting in substantially cone-shaped lens ends with smooth surfaces due to the surface tension in the core material.
The following references are cited to show related art: P. Kayoun, C. Puech, M. Papuchon, H. J. Arditty, "Improved Coupling Between Laser Diode and Single-Mode Fiber Tipped With A Chemically Etched Self-Centered Diffracting Element," Electronics Letters, Vol. 17, No. 12, pp. 400-402, 1981; M. Kawachi, T. Edahiro, H. Toba, "Microlens Formation on VAD Single-Mode Fiber Ends," Electronics Letters, Vol. 18, No. 2, pp. 71-72, 1982; D. S. Alles, "Trends in Laser Packaging," 40th Electronic Components and Technology Conference, May 20-23, 1990; and C. J. Sun, S. Sumida, S. Sakaguchi, T. Miyashita, "Circular Channel Waveguides and Lenses Formed From Rectangular Channel Waveguides," U.S. Pat. No. 5,064,266, 1991.